ath9k: Add recovery mechanism for hw TSF timer

Configure the TSF based hardware timer for a channel switch.
Also set up backup software timer, in case the gen timer fails.
This could be caused by a hardware reset.

Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Signed-off-by: Rajkumar Manoharan <rmanohar@qti.qualcomm.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>